Vapor generator control

ABSTRACT

A vapor generating system comprising a pressure vessel wherein secondary liquid is vaporized through indirect heat exchange with a hot primary fluid and including a control arrangement whereby thermal shock stresses in the vessel are limited by controlled preheating of secondary liquid being fed thereto.

United States Patent [1 1 Sprague et al.

[ Nov. 13,1973

[ VAPOR GENERATOR CONTROL [751 Inventors: Theodore S. Sprague, Hudson;John Schlicting, Akron, both of Ohio; Bertrand N. McDonald, Lynchburg,Va.

[73] Assignee: The Babcock & Wilcox Company, New York, NY. VI

22 Filed: Apr. 26, 1972 [21] App1.No.: 247,543

52 U.S.Cl. ..122/32,122/7 [51] Int. Cl. F22b 1/02 [58] Field of Search122/32, 33, 34, 7 R

[56] References Cited UNITED STATES PATENTS 3,070,536 12/1962 Taylor etal. 122/32 X HOT FLUID OUPCE Murray et al. 122/33 l-lryniszak et al.122/33 X Primary ExaminerKenneth W. Sprague Att0rneyJ. Maguire [57]ABSTRACT A vapor generating system comprising a pressure vessel whereinsecondary liquid is vaporized through indirect heat exchange with a hotprimary fluid and including a control arrangement whereby thermal shockstresses in the vessel are limited by controlled preheating of secondaryliquid being fed thereto.

10 Claims, 2 Drawing Figures souRceg PAIENYEDNHV 13% 3,771,497

SHEEI 18F 2 HOT FLUID 44 SOURCE H FEED WATER SOURCE PMENIEDnuvm r9753.771.497

sum 20? 2 55 FIG. 2 Y HOT FLUID 45 AUX. HOT SOURCE Li FLUID 4 SOURCE HA56 51 FEED WATER 59 48 SOURCE 5 VAPOR GENERATOR CONTROL BACKGROUND OFTHE INVENTION This invention relates in general to the control of vaporgenerators, and more particularly to a control arrangement for limitingthe thermal shock stresses in the vapor generator pressure vessel bycontrolled preheating of secondary liquid fed into the vessel forvaporization therein by heat extracted from a hot primary fluid.

The instant invention has particularly advantageous use in connectionwith nuclear powered vapor generators. In many known nuclear poweredvapor generators, heat is extracted from the reactor core by a primaryfluid passed therethrough and circulated in a closed cycle through tubestransiting a pressure vessel. The heat extracted from the core isreleased to vaporize secondary liquid that is fed into the vessel inphysically separated heat exchange with the primary fluid.

One of the problems encountered in the prior art is that of maintainingstructural integrity of the pressure vessel against thermal shocksthereto occasioned by the introduction into the vessel of excessivelycold secondary liquid. Under various operating conditions, the primaryfluid temperature may be relatively high compared to the incomingsecondary fluid temperature, and where the primary fluid flows throughtubes received in a tubesheet that is also exposed to incoming secondaryliquid, severe thermal shock stresses to the tubesheet may result. It istherefore important that the temperature differential between thesecondary liquid feed to the vessel and the primary fluid leaving thevessel be held within a prescribed safe limit.

SUMMARY OF THE INVENTION According to the invention, thermal shocks tothe vessel are reduced in severity by supplementary preheating of thesecondary liquid outside the vapor generator pressure vessel. Thepreheat temperature of the secondary liquid is automatically controlledby regulating means comprising a heat exchanger and control apparatuslocated outside the pressure vessel. The heat exchanger is disposed toreceive a heat input from the appropriate one of a plurality of heatsources for heating the secondary liquid entering the vessel and thecontrol apparatus includes series flow connected first and second valvesinterposed between the heat exchanger and the heat sources. The firstvalve being operable to select the heat source capable of supplying theheat exchanger with a heat input capable of raising the temperature ofthe secondary liquid entering the vessel to a required value and thesecond valve being operable to vary the quantity of selected heat inputto the heat exchanger to maintain this required temperature value.

In the main embodiment of the invention, the control apparatus includesmeans for determining the temperature of the secondary liquid at theinlet to the vessel and comprises pressure sensing means fortransmitting the pressure of heating fluid from within the heatexchanger to a controller which in turn positions a flow control valvefor regulating the quantity of heating fluid being admitted to the heatexchanger. j

An alternate embodiment of the invention includes means for determiningthe temperatures of the primary fluid at the outlet of the vessel andthe secondary liquid at the inlet to the vessel, respectively. Thetemperature determining means include a first transducer disposed tosense the primary fluid temperature at the outlet of the vessel andestablish a signal representative thereof and a second transducerdisposed to sense the secondary liquid temperature at the inlet to thevessel and establish a signal representative thereof. The twotemperature signals are transmitted to a controller which emits anoutput signal equivalent to the difference between the two inputsignals. The output signal is transmitted to a flow control valve forregulating the quantity of heating fluid being admitted to the heatexchanger.

In both the main and alternate embodiments of the invention, the controlapparatus includes means for determining the available heat input andcomprises pressure sensing means for transmitting the pressure ofheating fluid from within one of the heat sources, normally designatedas the primary heat source, to a controller which in turn positions afluid selector valve to admit fluid from the appropriate one of theheating sources. The selector valve is controlled in accordance to apredetermined heating fluid pressure value and is normally positioned toadmit fluid from the primary heat source, however, if fluid within theprimary source drops below the predetermined value, the selector valveis repositioned to admit fluid from an auxiliary heat source.

The fluid selector and control valves are regulated to maintain theheating fluid pressure within the heat exchanger at a value which has asaturation temperature substantially equal to the required secondaryliquid temperature at the inlet to the vessel thereby maintaining thetemperature difference, between the entering secondary liquid and theprimary fluid leaving the vessel, within a given limit.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a sectional side elevationof the vapor generating vessel and the heat exchanger and includes aschematic showing of various components associated with the mainembodiment of the invention.

FIG. 2 is a sectional side elevation of the heat exchanger and includesa schematic showing of various components associated with an alternateembodiment of the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION In FIG. 1 of thedrawing there is shown a vapor generator control arrangement in which ahot primary fluid such as the coolant from a nuclear reactor core (notshown), is passed through a generally upright pressure vessel 10 andtherein undergoes physically separated heat exchange with a secondaryliquid, such as water, fed into vessel 10. Primary fluid enters into aplenum chamber 11 at one end of vessel 10 and passes through tubes 12(shown in centerline layout only) received in tubesheets l3 and 14,collects in a plenum chamber 15 at the opposite end of vessel 10wherefrom it exits for recirculation.

Within vessel 10 there is a shroud 16 surrounding the bundle of tubes 12and open at both ends. A ring plate 17 connected at its inner edge toshroud 16 and at its outer edge to the wall 18 of vessel 10 serves toseparate incoming feedwater introduced through a nozzle 19 from outgoingvapor exiting through another nozzle 20.

Feedwater entering through nozzle 19 is constrained by plate 17 to flowdownward in the annular space 21 between wall 18 and shroud 16, andthence into the open lower end 22 of shroud l6, sweeping across thetubesheet 14. The lower interior portion of shroud 16 is designed toserve as an integrally contained economizer chamber 23, and for suchpurpose there are provided baffles 24, 25, 26 that increase theresidence time of the feedwater in chamber 23. The feedwater is heatedand vaporized by heat transfer through tubes 12 from the primary fluid.The vapor thus produced, which can be either saturated or superheateddepending upon the amount of heating provided by tubes 12, passes outthe open upper end 27 of shroud 16 and flows down the annular passage28, between shroud 16 and wall.l8 and above ring 17, for exit throughnozzle 20.

The primary fluid flowing through tubes 12 is at a considerably highertemperature, as much as several hundred degrees higher than thefeedwater supply temperature, such that, if the feedwater from thesource S were to be introduced directly into vessel 10, severe thermalshock stresses would occur because the relatively cold feedwater wouldchill the side of tubesheet 14 adjacent the shroud end 22 whereas theother side of tubesheet 14 would be heated by the primary fluid inplenum 15.

To avoid such thermal shock stresses and the problems they create, theinvention proposes to preheat the feedwater before entry into vesselsuch that the difference, between the temperature of the incomingfeedwater flowing through nozzle 19 and the temperature of the primaryfluid in plenum 15, is maintained within a given limit. I

In accordance with the present invention, feedwater source S is flowconnected with nozzle 19 through a heat exchanger 30 that effectspreheating of the feedwater for the purpose of maintaining the specifiedlimit in temperature difference between the feedwater entering thevessel and the primary fluid leaving the vessel.

Heat exchanger 30 is expediently a shell and tube type heat exchanger,and has an inlet plenum 31 flow connected by a conduit 32 to the sourceS for receiving partially-heated or cold water therefrom. The feedwateris conveyed from the plenum 31 through a bundle of U-bent tubes 33wherein it is heated by indirect heat exchange with a hot fluid flowingthrough the shell side of heat exchanger 30. The preheated feedwaterexiting from the tubes 33 is collected in an outlet plenum 35 andthereafter delivered to nozzle 19 through a conduit 36 flow connectedtherewith.

The heating or hot fluid is shown generally as being supplied to heatexchange 30 from either a source H or an auxiliary source HA. The hotfluid from source H is conveyed through a conduit 46 to one of twoinlets of the selector valve 48, while the hot fluid from auxiliarysource HA is'conveyed through a conduit 45 to the other inlet of theselector valve 48. The selected hot fluid is thereafter conveyed intothe shell side of heat exchanger 30 through a conduit 47 which includesa control valve 49 for regulating the quantity of hot fluid beingadmitted into heat exchanger 30. The spent hot fluid is discharged fromheat exchanger 30 through a drain line 37. It will be understood thatthere may be more than one auxiliary heat source associated with theheat exchanger 30.

In the main embodiment of the invention, the selector and control valves48 and 49 are actuated by controllers 54 and 57, respectively. Thecontroller 54 is arranged to respond to a signal representing thepressure of hot fluid from within the primary heat source H; this signalbeing applied via transmission line 53 which interconnects the primaryheat source H with the input side of controller 54. The controller 54generates a control or output signal that is applied to the selectorvalve 48 via transmission line 58 which interconnects the output side ofcontroller 54 to the actuating device 44 associated with the valve 48.The controller 57 is arranged to respond to the pressure of hot fluidfrom within the heat exchanger 30; this pressure being applied viatransmission line 43 which interconnects heat exchanger 30 with theinput side of controller 57. The control or output signal fromcontroller 57 is applied to the flow control valve 49 via transmissionline 60 which interconnects the output side of controller 57 to theactuating device 34 associated with the valve 49.

The choice and details of the actual components used to control andactuate the valves 48 and 49 are left to the artisan, since there aremany known components that can be used. For example, the controllers 54and 57 may be of the type that include a set point representedschematically by arrow Y which provides the means for selecting the hotfluid pressure required to heat the entering feedwater to the desiredtemperature. The actuators 34 and 44 can be pressure responsive springand diaphragm operated mechanisms.

In the alternate embodiment of the invention as shown in FIG. 2, thereis provided a transducer 39 disposed to sense the temperature of thefeedwater entering vessel 10, and a transducer 40 disposed to sense thetemperature of the primary fluid leaving vessel 10. Each of thetransducers 39 and 40 establishes a signal representing the temperaturewhich it senses. The feedwater temperature signal T and the primaryfluid temperature signal T, are applied to a controller 38 viatransmission lines 41 and 42 from transducers 39 and 40, respectively.The controller 38 generates an output signal that is applied to the flowcontrol valve 49 via transmission line which interconnects the outputside of controller 38 to the actuating device 52 associated with thevalve 49. The controller 56 is arranged to respond to a signalrepresenting the pressure of hot fluid from within the primary heatsource H; the signal being applied via transmission line 55 whichinterconnects the primary heat source H with the input side ofcontroller 56. The controller 56 generates a control or output signalthat is applied to the selector valve 48 via transmission line 59 whichinterconnects the output side of controller 56 to the actuating device51 associated with the valve 48.

The choice and details of the actual components used to control andactuate the valves 48 and 49 are left to the artisan, since there aremany known components that can be used. For example, the controllers 38and 56 may be of the type that include a set point representedschematically by arrows X and Y, respectively. Set point X provides themeans for selecting the desired (Ty-Ty) limit which in turn regulatesthe hot fluid'pressure within the heat exchanger 30. Set point Yprovides the means for selecting the hot fluid pressure required forheating the entering feedwater to the desired temperature. The actuators51 and 52 can be electrically responsive devices mechanically linked toor integrated with the respective valves 48 and 49.

The materials used in the fabrication of the vapor generator,particularly, the lower tubesheet, will determine the limit onfeedwater-to-primary fluid temperature required throughout the entireoperating range of the unit. For example, it has been found thatcladding the lower tubesheet with Inconel will allow a steepertemperature gradient across the thickness of the tubesheet withoutinjurious effect to the metal.

In the operation of the invention, the temperature of the feedwaterentering vessel is maintained at a value or values which werepredetermined as satisfying the feedwater-to-primary fluid differentialtemperature limit prescribed for the steam generator throughout theentire load range. A power plant cycle is normally designed wherein thefeedwater entering the steam generating vessel is within a fewFahrenheit degrees of the saturation temperature of the heating fluidpassing through the highest stage heater, which is represented in ourpreferred embodiments as heat exchanger 30.

Accordingly, in both embodiments of the invention, the feedwatertemperature entering the vessel 10 is being maintained at thepredetermined value by regulating the hot fluid pressure within the heatexchanger to keep a pressure having a saturation temperaturecorresponding to the predetermined feedwater temperature. The hot fluidmay be obtained from any number of heat sources, for example, theprimary source H may be steam from an extraction point on the highpressure turbine (not shown) while the auxiliary source HA is steam fromthe outlet of the vapor generator.

In the main embodiment of the invention, the hot fluid saturationpressure required for maintaining the desired feedwater temperature isregulated through the controller 54 which senses the available hot fluidpressure from the primary heat source H, and the controller 57 whichsenses the working hot fluid pressure within the heat exchanger 30. Bothcontrollers are equipped with adjustable set points Y which provide themeans for selecting the hot fluid pressure required to heat the enteringfeedwater to the desired temperature. The controller 54 compares theamplitude of the input signal from the primary heat source H with theamplitude of the set point signal Y representing the selected pressureand, whenever, the input signal is smaller than the signal of set pointY, an output signal is transmitted from controller 54 to the actuator 44which then positions the valve 48 to admit steam from the auxiliary heatsource HA. The controller 57 compares the amplitude of the input signalfrom heat exchanger 30 with the amplitude of the set point signal Yrepresenting the selected pressure and, whenever, there is a differencebetween the input and set point signals, an output signal is transmittedfrom controller 57 to the actuator 34 which adjusts the control valve 49to increase or decrease the flow of hot fluid to the heat exchanger 30,as required.

In the alternate embodiment of the invention, the hot fluid saturationpressure required for maintaining the desired feedwater temperature isregulated through the controller 56 which senses the available hot fluidpressure from the primary heat source H, and the controller 38 whichsenses the temperature of the feedwater entering vessel 10 and thetemperature of the primary fluid leaving vessel 10. The controllers 56and 38 are equipped with adjustable set points Y and X which provide themeans for selecting the hot fluid pressure required to heat the enteringfeedwater to the desired temperature. The controller 56 compares theamplitude of the input signal from the primary heat source H with theamplitude of the set point signal Y representing the selected pressureand, whenever, the input signal is smaller than the signal of set pointY, an output signal is transmitted from the controller 56 to theactuator 51 which then positions the valve 48 to admit steam from theauxiliary source HA. The controller 38 resolves the primary fluid andsecondary fluid input signals into a signal representative of thetemperature difference therebetween and compares the latter signal withthe signal of set point X and whenever a difference exists between theset point and resolved signals, an output signal is transmitted fromcontroller 38 to the actuator 52 which adjusts the control valve 49 toincrease or decrease the flow of hot fluid to the heat exchanger 30, asrequired.

While in accordance with provisions of the statutes there is illustratedand described herein a specific embodiment of the invention, thoseskilled in the art will understand that changes may be made in the formof the invention covered by the claims, and that certain features of theinvention may sometimes be used to advantage without a corresponding useof the other features.

We claim:

1. A vapor generating system having a pressure vessel, a primary fluidand a secondary liquid flowing in physically separated heat exchangethrough said vessel wherein heat extracted from the primary fluid isused to vaporize the secondary liquid, the improvement comprising meansfor regulating the temperature of the secondary liquid entering thevessel including a heat exchanger and a plurality of hot fluid sourceslocated outside the vessel, the heat exchanger being disposed to receivehot fluid from a selected one of said sources for heating the secondaryliquid entering the vessel, control apparatus for selecting said sourceand adjusting the hot fluid input rate to the heat exchanger tocorrespondingly adjust the temperature of the secondary liquid enteringthe vessel thereby maintaining the temperature difference between theentering secondary liquid and the primary fluid leaving the vesselwithin a given limit.

2. The method of operating a vapor generating system including apressure vessel, a primary fluid and a secondary liquid flowing inphysically separated heat exchange through said vessel wherein heatextracted from the primary fluid is used to vaporize the secondaryliquid, a heat exchanger and a plurality of hot fluid sources locatedoutside the vessel, the heat exchanger being disposed to receive hotfluid from a selected one of said sources for heating the secondaryliquid entering the vessel, and comprising the steps of:

sensing the hot fluid pressure in at least one of said sources,

selecting the source in accordance with said sensed pressure,

determining the temperature of the secondary liquid in at least onegiven location, and

varying the hot fluid input rate to the heat exchanger in accordancewith said determined temperature to maintain the temperature differencebetween the entering secondary liquid and the primary fluid leaving thevessel within a given limit.

3. The improvement according to claim 1 wherein the control apparatusincludes serially disposed first and second valves intermediate of andin flow communication with said hot fluid sources and heat exchanger,the first valve being operable for selecting the source and the secondvalve being operable for varying the rate of hot fluid being admittedinto the heat exchanger, the first valve being upstream fluid flow-wiseof said second valve.

4. The improvement according to claim 1 wherein the control apparatusincludes separate means for determining the hot fluid pressure and thesecondary liquid temperature in at least one given location.

5. The improvement according to claim 3 wherein the means fordetermining the hot fluid pressure includes pressure sensing meansoperatively associated with at least one of said sources.

6. The improvement according to claim 4 wherein the means fordetermining the secondary liquid temperature includes pressure sensingmeans operatively associated with said heat exchanger.

7. The improvement according to claim 4 wherein the means fordetermining the secondary liquid temperature includes a transducerdisposed to sense the temperature of the secondary liquid entering thevessel and establish a signal representative thereof.

8. The improvement according to claim 4 including means for determiningthe temperature of the primary fluid in at least one given location.

9. The improvement according to claim 8 wherein the means fordetermining the primary fluid temperature includes a transducer disposedto sense the tem perature of the primary fluid exiting the vessel andestablish a signal representative thereof.

10. The improvement according to claim 2 including the steps ofdetermining the temperature of the primary fluid in at least one givenlocation and varying the hot fluid input rate to the heat exchanger inaccordance with said determined primary fluid temperature.

1. A vapor generating system having a pressure vessel, a primary fluidand a secondary liquid flowing in physically separated heat exchangethrough said vessel wherein heat extracted from the primary fluid isused to vaporize the secondary liquid, the improvement comprising meansfor regulating the temperature of the secondary liquid entering thevessel including a heat exchanger and a plurality of hot fluid sourceslocated outside the vessel, the heat exchanger being disposed to receivehot fluid from a selected one of said sources for heating the secondaryliquid entering the vessel, control apparatus for selecting said sourceand adjusting the hot fluid input rate to the heat exchanger tocorrespondingly adjusT the temperature of the secondary liquid enteringthe vessel thereby maintaining the temperature difference between theentering secondary liquid and the primary fluid leaving the vesselwithin a given limit.
 2. The method of operating a vapor generatingsystem including a pressure vessel, a primary fluid and a secondaryliquid flowing in physically separated heat exchange through said vesselwherein heat extracted from the primary fluid is used to vaporize thesecondary liquid, a heat exchanger and a plurality of hot fluid sourceslocated outside the vessel, the heat exchanger being disposed to receivehot fluid from a selected one of said sources for heating the secondaryliquid entering the vessel, and comprising the steps of: sensing the hotfluid pressure in at least one of said sources, selecting the source inaccordance with said sensed pressure, determining the temperature of thesecondary liquid in at least one given location, and varying the hotfluid input rate to the heat exchanger in accordance with saiddetermined temperature to maintain the temperature difference betweenthe entering secondary liquid and the primary fluid leaving the vesselwithin a given limit.
 3. The improvement according to claim 1 whereinthe control apparatus includes serially disposed first and second valvesintermediate of and in flow communication with said hot fluid sourcesand heat exchanger, the first valve being operable for selecting thesource and the second valve being operable for varying the rate of hotfluid being admitted into the heat exchanger, the first valve beingupstream fluid flow-wise of said second valve.
 4. The improvementaccording to claim 1 wherein the control apparatus includes separatemeans for determining the hot fluid pressure and the secondary liquidtemperature in at least one given location.
 5. The improvement accordingto claim 3 wherein the means for determining the hot fluid pressureincludes pressure sensing means operatively associated with at least oneof said sources.
 6. The improvement according to claim 4 wherein themeans for determining the secondary liquid temperature includes pressuresensing means operatively associated with said heat exchanger.
 7. Theimprovement according to claim 4 wherein the means for determining thesecondary liquid temperature includes a transducer disposed to sense thetemperature of the secondary liquid entering the vessel and establish asignal representative thereof.
 8. The improvement according to claim 4including means for determining the temperature of the primary fluid inat least one given location.
 9. The improvement according to claim 8wherein the means for determining the primary fluid temperature includesa transducer disposed to sense the temperature of the primary fluidexiting the vessel and establish a signal representative thereof. 10.The improvement according to claim 2 including the steps of determiningthe temperature of the primary fluid in at least one given location andvarying the hot fluid input rate to the heat exchanger in accordancewith said determined primary fluid temperature.